Energy absorbing materials

ABSTRACT

A novel elastomer having a very low compression set and a very slow recovery from compression comprises a flexible polyurethane of essentially linear structure containing unsatisfied hydroxyl groups, and is the reaction product of a substantially linear polyol having hydroxyl end groups and a molecular weight in the range 6000 to 12000, with an isocyanate in less than stoichiometric amount. This elastomer can be used as one layer of an energy-absorbing material, in combination with a more difficultly compressible layer comprising a tough flexible polymeric matrix having a plurality of rigid hollow bodies embedded therein.

This application is a continuation-in-part of my application Ser. No.681,528 filed Apr. 29, 1976, now abandoned.

This invention relates to energy absorbing materials, and moreparticularly but not exclusively to energy absorbing materials suitablefor use in automobile bumpers and other devices intended to provideprotection against damage due to impact, shock or collision, and for usein the absorption of sound.

It has been proposed to manufacture automobile bumpers frommicro-cellular urethane elastomers by moulding techniques. These have anumber of advantages over conventional metal bumpers, in particular theycan withstand multiple impacts without functional or visual damage andare much lighter in weight. However, bumpers formed from micro-cellularurethane elastomers only provide useful protection against impacts atvery low speeds, at the very most up to about 10 miles an hour.

An object of the present invention is to provide an energy absorbingmaterial which can provide an improved protection against damage due toimpact, shock or collision.

A further object of the invention is to provide a solid energy-absorbingmaterial with quasi-liquid properties.

A further object is to provide a solid elastomer having very slowrecovery from deformation due to an applied force, for use in energyabsorption.

The present invention in one aspect provides an energy absorbingmaterial which comprises an elastomeric layer of polymeric materialhaving a low compression set and a delayed recovery from compression, incombination with a more difficulty compressible layer comprising a toughflexible polymeric matrix having a plurality of rigid hollow bodiesembedded therein, the arrangement being such that under the action ofdeforming forces deformation of the layers takes place sequentially.

Preferably the energy absorbing material is encased in an outer skinwhich is resistant to abrasion and weathering.

The invention in another aspect provides an elastomer comprising aflexible polyurethane having a compression set less than 5%, anelongation at break of 500 to 1200%, and a recovery which is delayedafter compression by at least 0.7 sec, and having a low branch molecularweight and a very low degree of cross-linking.

The invention in another aspect provides an elastomer comprising aflexible polyurethane of essentially linear structure containingunsatisfied hydroxyl groups, and having a compression set less than 15%,an elongation at break of at least 500%, and a recovery which is delayedafter compression by at least 0.7 sec.

The invention further provides a method of making a polyurethaneelastomer having a low compression set and a delayed recovery fromcompression, which method comprises reacting a slightly branched polyolof low molecular weight with a relatively small amount of an isocyanate.

The invention further provides a method of making a polyurethaneelastomer having a low compression set and a delayed recovery fromcompression, which method comprises reacting a substantially linearpolyol having hydroxyl end groups and a molecular weight in the range6000 to 12,000 with an isocyanate in less than stoichiometric amountwhereby the resulting elastomer contains unsatisfied OH groups.

The invention further provides a method of making a foam elastomercomprising admixing a polyurethane elastomer with a substantially linearpolyol having a molecular weight in the range 6000 to 12,000 and anisocyanate in less than stoichiometric amount with respect to thepolyol, and a foaming agent.

In the following description of the invention, wherein further objectsand advantages thereof are revealed, reference will be made to theaccompanying drawing, which shows a schematic cross-section through anenergy-absorbing material embodying the invention.

The drawing shows (not to scale) a section through an energy-absorbingmulti-layer material applicable to an automobile bumper.

The material essentially comprises a layer 1 of elastomeric non-cellularpolyurethane having a low compression set (less than 5%) and a delayedrecovery from compression (longer than 0.7 sec.) bonded to a lesscompressible layer 2 comprising a tough flexible polyurethane matrix 6in which a multiplicity of rigid hollow bodies 3 are embedded. Thelayers 1, 2 are encased in an abrasion and weather-resistant skin 4 andthe material is mounted on a metal backing plate 5. In an impact, theelastomeric layer 1 will deform first and this layer will absorb alllow-energy impacts. Heavier impacts deform the less compressible layer 2and, if heavy enough, fracture the hollow bodies 3.

Further particulars of the constituents of the energy-absorbing materialare set forth below. Although the invention is particularly describedwith reference to the use of the new energy absorbing materials in theproduction of automobile bumpers, it is to be understood that theinvention is not limited thereto, and for example the new energyabsorbing materials may find application as shock absorbers and in thedamping of machinery.

Although the invention is not restricted to any particular theory, it isbelieved that automobile bumpers are subjected to two main types ofserious damage. These are:

(1) Low speed impacts caused by parking errors, mainly involvingcollision with stationary objects, and

(2) High speed impacts in road accidents.

The elastomeric layer 1 is provided to reduce damage due to low speedimpacts. In order that the bumper should appear substantially unalteredafter such an impact the elastomeric layer should have a low compressionset, for example less than 10%, preferably less than 5%, and mostpreferably less than 1%. In this specification the compression set isdefined as the percentage lack of recovery after compression. As a roughapproximation, the average impact time of an automobile collision is ofthe order of 0.7 secs., and for use in a bumper the recovery delay ofthe elastomeric layer after removal of applied compression force must begreater than this impact time. Polymeric materials having a recoverytime of at least 2 secs. are desirable, and a preferred recovery time isbetween 2 and 10 secs. Preferably the hysteresis pattern of thepolymeric material should show an initial slow rate of recovery. Asuitable polymeric material for the elastomer is a flexible non-cellularpolyurethane of essentially linear structure containing unsatisfiedhydroxyl groups, having a compression set less than 15%, preferably lessthan 5%, an elongation at break of at least 500%, and a recovery whichis delayed after compression by at least 0.7 sec. Such polyurethaneelastomers are novel and accordingly this invention also provides, as anew composition of matter, an elastomeric polyurethane having theaforesaid structure and properties.

The elastomer preferably has a hardness, on the Shore 00 scale, notexceeding 50, preferably not exceeding 20, preferably in the range 0 to10.

Typical polyurethane elastomers of the invention have an elongation atbreak preferably exceeding 600%, e.g. about 800%; a tear strength of 5to 20 lbs./linear inch, particularly 5 to 10 lbs./linear inch; and atensile strength up to 50 lbs./square inch. The rather low tear strengthand tensile strength of such materials can be counteracted byincorporating fibrous material.

In addition to the above properties, the elastomer should also be stableat temperatures of from -40° C to +100° C.

Suitable polyurethane polymers for the elastomer are those having a lowbranch molecular weight and a very low degree of cross-linking. Such apolyurethane may be produced, for example, by reacting a low molecularweight linear or slightly branched polyol with a relatively small amountof an isocyanate e.g. methyl diisocyanate or an aryl isocyanate such asmethylene diisocyanate or 4,4'-diphenylmethane diisocyanate. The chosenisocyanate must be at least as reactive to hydroxyl group as methylenediisocyanate, and examples are toluene diisocyanate, methylenediisocyanate (the preferred one) or triphenyl methyltriisocyanate. Theisocyanate may if desired be mixed with a diluent, for example methylenechloride. The polyol should have a molecular weight of from 6000 to12000, preferably 7000 to 9000, and may be prepared by condensation of apolyglycol, in particular a polyalkylene glycol such as polyethyleneglycol or polypropylene glycol, to a molecular weight of between 6000and 12000. The polyol has hydroxyl end groups, preferably only two OHgroups/molecule, and is essentially linear with the minimum ofbranching. The polyol also may be prepared by heating a suitablepolyester in an autoclave under pressure at a temperature of from about160 to 250° C for a period of up to about 8 hours. Very good resultshave been obtained using a polyol designated PM 515X or PM 735X andsupplied by Bostik Limited.

The isocyanate and the polyol are reacted together using standardurethane technology, in the complete absence of water and using asuitable catalyst. Triethylene diamine is the preferred catalyst butother tertiary amines are satisfactory. The isocyanate is present inless than the stoichiometrical quantity needed to react with thehydroxyl groups, so that not all of the hydroxyl groups are satisfied.The resulting polymer is believed to have foreshortened chains becausethe polymerization cannot proceed to completion, with a minimum of chainbranching. The resulting solid polymer behaves like a quasi-liquid,being readily deformed by an applied force and slow to recover, althoughin the absence of such a force it takes up a defined shape and volume.

It is believed that, to achieve the desired physical properties of thematerial, the polyurethane elastomer should contain 0.002 to 0.004 gramof unsatisfied OH groups per gram of elastomer, preferably 0.0023 to0.0034 gm OH/gram. To achieve this the mole ratio of OH to NCO in thereactants should be in the range 5:1 to 1.22 : 1 corresponding toapproximately 80% to 55% unsatisfied OH groups in the product.

Certain properties of the elastomer, in particular tensile strength,tear strength, elongation and compression set, can be improved bycarrying out the reaction under superatmospheric pressure, for examplein the range 50 to 150 psi. This is accompanied by a small increase inhardness.

The molecular weight of the polyol is important with respect to theenergy absorbing properties of the material since in general below amolecular weight of 6000 the polymer material will suffer permanentdeformation and above 12000 the polymer will recover too quickly from anapplied force (i.e. with a delay less than 0.7 sec.).

Fillers may be added to stiffen the material. Hydrocarbons may be addedas a diluent during polymerized by up to 10% by weight of the polyol toreduce the surface tack of the finished polymer.

It has been found that surface tack can be reduced and abrasionresistance increased by the incorporation of a small amount of asilicone polycarbinol, in particular a polypropylene oxide -- siloxanecopolymer. Normally such additives are present at over 2% by weight ofthe polyol, but such amounts are ineffective in the elastomer of theinvention; instead, amounts less than 2%, preferably 0.5% to 1%, areeffective in the elastomer of the invention and improve both surfacetack and abrasion resistance.

Examples of specific elastomers of the invention and their manufacturewill now be given.

Table 1 lists four different reaction mixtures A to D each of which waspolymerized at atmospheric pressure and also at 80 psia so that in alleight different products were obtained. The physical properties of eachof these products are listed in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                               SAMPLE A SAMPLE B SAMPLE C EXAMPLE D                                          Atmos-   Atmos-   Atmos-   Atmos-                                             pheric   pheric   pheric   pheric                                       Units pressure                                                                           80 psi                                                                            Pressure                                                                           80 psi                                                                            Pressure                                                                           80 psi                                                                            pressure                                                                           80                 __________________________________________________________________________                                                               psi                Chemical Composition                                                          Polyol               ppw   21.25    20.75    20.50    20.25                   Polypropylene oxide - Siloxane copolymer                                                           ppw   0.16     0.14     0.13     0.12                    Methyl diisocyanate (86% pure)                                                                     ppw   1.00     1.00     1.00     1.00                    Physical Properties                                                           Hardness (40 hrs)    Shore'00'                                                                           8    10  18   20  35   38  40   42                 Density              gm/cc 1.34 1.34                                                                              1.34 1.34                                                                              1.34 1.34                                                                              1.34 1.34               Ultimate tensile strength                                                                          psi   14   18  18   21  24   30  40   45                 Elongation at break  %     800  900 720  840 680  700 640  650                Tear Strength        lb/linear"                                                                          5.0  5.9 5.5  6.4 6.3  6.9 6.8  7.6                Compression set at + 22° C                                                                  %     4    0   13   0   13   1   13   1                  Compression set at - %40° C                                                                       5    0   14   0   14   1   14   2                                             no   no  no   no  no   no  no   no                 Impact (.5 lb ball at 6ft)                                                                         --    crack                                                                              crack                                                                             crack                                                                              crack                                                                             crack                                                                              crack                                                                             crack                                                                              crack              Complex modulus phase shift                                                                        ° C/decade                                                                            20   22                                   Recovery delay       secs  2.0  2.0 1.5  1.5 1.0  1.0 1.0  1.0                __________________________________________________________________________

Each mixture consisted of the same linear polyol Bostik PM 735X, ofmolecular weight 7000 - 9000 (determined by measurement of the hydroxylnumber), based on polypropylene glycol. This polyol contained 0.7% to 2%triethylene diamine as catalyst. It was placed in a glass vessel withthe methyl diisocyanate and the polypropylene oxide -- siloxanecopolymer, at 20° C, and the mixture stirred for 20 sec. Polymerizationtook 1 to 20 min. according to the proportion of catalyst present. With2% catalyst there was a noticeable viscosity increase after 60 sec.,gelation occurred in 4 min. and the material was solid after 8 minutes,whereafter it could be removed from the vessel or mould. Heat wasevolved, raising the temperature to as much as 80° C.

In the product, 65% of the original OH groups remain unsatisfied,corresponding to 0.0028 gm OH per gram of product.

The quantities of the constituents are given in parts by weight (ppw).It will be seen that a reduction in the proportion of polyol leads to anincrease in hardness, tensile strength and tear strength but reduces theelongation and recovery delay time after compression, and increases thecompression set. Polymerization under pressure also increases hardnessand strength but increases the elongation and reduces compression set.

The maintenance of low compression set at low temperatures is to benoted. Flexibility is also maintained: a sample 10 inches by 0.5 inchesby 0.25 inches was kept at -40° C for 24 hours; it could then be wrappedaround a mandrel of 3 inches diameter without cracking. The materialalso withstands the impact test without cracking, at -40° and 75° C. Thesoftening temperature depends somewhat on the formulation andpolymerization conditions but is typically in the range 90° and 120° C.

The recovery delay was determined from dynamic measurements of thecomplex modulus phase shift, showing substantial recovery after 2 to 3sec. (Sample A) and complete recovery after 100 sec.

The material is chemically and dimensionally stable, with goodresistance to water, ozone, oil, petrol and ethylene glycol.

The impact-absorbing properties of the elastomer were investigated bythe Lupke (BS 903) pendulum rebound test. Table 2 compares a specimen(LCS) of the elastomer of the invention (layer 1 of the material shownin the drawing). It can be seen that this elastomer is "dead".

                  TABLE 2                                                         ______________________________________                                        LUPKE PENDULUM at 20° C                                                                 HARDNESS   REBOUND RE-                                       MATERIAL         (IRHD)     SILIENCE %                                        ______________________________________                                        Natural rubber   52         69                                                Butyl            45         13                                                SBR (Styrene butadiene                                                                         53         34                                                rubber)                                                                       Nitrile          57         32                                                EPDM (Ethylene propylene                                                      elastomer)       53         48                                                Neoprene         62         57                                                Silicone         53         42                                                "Viton" fluorinated rubber                                                                     72          5                                                LCS              less than 1                                                                               0                                                ______________________________________                                    

The present elastomer has excellent sound attenuation and vibrationdamping and is useful as a sound-deadening material e.g. in vehicles.Its sound attenuation is much greater than that of materials commonlyused hitherto, as is shown by the comparative Table 3.

                  TABLE 3.                                                        ______________________________________                                        Material       Thickness (mm)                                                                             Output, decibels                                  ______________________________________                                        Bestobell "Acoustolan"                                                                       62.5         80                                                Neoprene foam  12.5         91                                                Neoprene rubber                                                                              10           81                                                Monothane polyurethane                                                                       35           66                                                "              10           76                                                Nitrile rubber 15           69                                                "              10           72                                                LCS Sample B   12.5         56                                                ______________________________________                                    

The foregoing description and in particular the numerical values ofphysical properties relate to the solid elastomer. However the elastomercan readily be produced in foam from e.g. by the addition of water andmethyl diisocyanate to react with the water, for example in theproportions 6 ppw water, 8 ppw methyl diisocyanate, 100 ppw polyol. Thewater preferably has a pH greater than 7. A 6-fold volume increase canbe attained. The foam produces greater rebound than the solid material,but much less than conventional polyurethane foam, as shown in Table 4below:

                  TABLE 4                                                         ______________________________________                                        Lupke pendulum test, sample thickness 12.5 mm.                                Material            % Rebound resilience                                      ______________________________________                                        Foam LCS (relative density 0.33)                                                                  12                                                        Neoprene foam       44                                                        Natural rubber foam 32                                                        Polyurethane foam   38                                                        ______________________________________                                    

The more difficultly compressible layer 2 is provided in order to absorbenergy generated by impact at higher speeds. It comprises a toughflexible polymeric matrix 6 having a plurality of rigid hollow bodies 3embedded therein. The physical properties of the polymer matrix arepreferably as follows:

    ______________________________________                                        Tensile strength:                                                                             500 to 3500 lbs. per square                                                   inch                                                          Elongation to break:                                                                          200 to 600%                                                   Tear strength:  120 to 150 lbs. per linear                                                    inch                                                          ______________________________________                                    

Preferably the polymeric matrix should be stable in the range of -40° Cto +100° C.

The polymer matrix 2 is preferably also formed from a polyurethanepolymer, although other polymers may also be used, for example a rubbermodified polystyrene, a polyolefin, a flexible polyester, an epoxy resinor polyvinyl chloride. Where the polymer matrix is formed from apolyurethane polymer, this is preferably a polyurethane produced byreaction of a polyester polyol having, for example, a molecular weightof 500 to 1000 with an aryl isocyanate such as 4,4'-diphenylmethanediisocyanate. Suitable polyurethanes are sold by Bostik Limited anddesignated GC148 and GC 155.

It is believed that under impact conditions, the energy absorbingproperties of the difficultly compressible layer 2 are due primarily tofracture of the rigid hollow bodies 3. A number of very hard materialscan be produced in the form of hollow bodies of generally sphericalshape, including phenolic resins, glass, silica and carbon. Preferablythe average diameter of the hollow bodies is within the range of from 50to 400 microns. The optimum quantity of rigid hollow bodies in thedifficultly compressible layer will depend to some extent upon theapplication, but will usually be in the range of from 10 to 60% byweight based on the total weight of the difficultly compressible layer.

In addition to the more serious damage, bumpers are also subject tominor scuffs, scratches, and abrasions, and against these the impactabsorbing material is preferably provided with a tough outer skin 4which, in addition to its abrasion resistance, should also be resistantto degradation by environmental agencies such as ultraviolet light,water, road salts, ethylene glycol, and automotive fuels. The outer skinshould also be stable at temperatures from -40° C to +100° C. The outerskin is preferably formed from a polymer having the following physicalproperties:

Tensile strength: 500 to 1500 lbs. per square inch

Elongation to break: 200 to 800%

Tear strength: 95 to 150 lbs. per linear inch

Hardness: 50 to 80 (Shore A scale)

Abrasion resistance: 0.2 cc per 1000 revs. (measured on a DuPont wheel)

Compression set: 12 to 25%

In addition to the above properties, the outer skin should preferably beresilient and have a good recovery.

Suitable polymers for use in the production of the outer skin includepolyvinyl chloride, synthetic and natural rubbers, polyolefins, andpolydienes although preferably the outer skin is also formed from apolyurethane polymer. The preferred polyurethane polymers are thoseprepared by reaction of a polyester polyol with an aryl isocyanate suchas 4,4'-diphenylmethane diisocyanate. Polyester polyols which have beenfound to give good results are those manufactured and sold by BostikLimited under the trade names PM 117X and PM 260X. These are believed tobe polyoxyalkyleneglycols having a molecular weight of 1100 to 1300 anda hydroxyl number of from 120 to 140.

Preferably, for an automobile bumper, the energy absorbing material ismounted upon a rigid e.g. metal backing plate 5.

Bumpers according to the present invention may be made by conventionalmoulding or casting procedures. The relative thicknesses of the energyabsorbing layers are dependent on the use. In a bumper, by way ofexample, the elastomeric layer 1 may be from 3 to 6 inches in thickness,and the more difficultly compressible layer 2 from 3 to 6 inches inthickness. Where an outer skin 4 is present this is preferably of from1/16 to 1/2 inch in thickness. Preferably the energy absorbing materialis arranged such that the more difficultly compressible layer 2 isadjacent to the backing plate 5, although this is not essential. Skin 4can be omitted between the energy-absorbing material and the backingplate 5.

Impact absorbing materials according to the invention can affordexcellent protection against impact for a wide variety of applications.In addition, the materials of the individual elastomeric and moredifficultly compressible layers may be used independently in appropriateenergy absorbing situations with excellent results, for example theelastomeric polyurethane polymers of the invention may find applicationin sports wear such as shin pads and athletic shoes, crash helmets,orthopaedic beds and shock absorber inserts, and in sound absorption.

It has also been found that the polyurethane polymer of the inventioncan be used to modify conventional solid polyurethane elastomers, forexample those having a molecular weight of between 500 to 4000, in sucha way that the coefficient of restitution of the elastomer is decreased.Such a combination, when foamed in the usual way, can be used to make aball suitable for sports and games, in particular the game of squashrackets. A ball, so made, is at least as good as and in many wayssuperior to a conventional hollow moulded rubber ball.

In order to achieve the correct bounce characteristic, which differswith each grade of ball, it is necessary to vary the amount of eachcomponent of the material. For example, as little as 25% of the polyoldescribed may be used or as much as 75%. To achieve othercharacteristics it may be necessary to add from 5% to 95%.

For example, 25 parts by weight of the described polyol are added to 63parts of a conventional polyurethane elastomer having a molecular weightat 2200, such as Bostik 2305, in a suitable glass vessel. 1 part ofwater is added and the whole thoroughly mixed by hand for 20 seconds. Tothis is added 11 parts of 4:4 methyl diisocyanate and the mixturestirred for 10 seconds. The mixture is then poured into one half at asuitable mould (at 18° C) and the other half placed on top. The mixtureis allowed to foam and fill the cavity. After 2 minutes the moulded foamball may be removed from the cavity. It is found that such a ball hasthe bounce characteristics of a conventional squash ball described aswhite spot grade, and satisfies the official requirements namely adiameter of 1 7/16 to 1 5/8 (39.65 to 41.28 mm) and weight of 13.1 to13.8 drams (23.29 to 24.66 gm).

I claim:
 1. An energy-absorbing material which comprises an elastomerlayer of polymeric material having a low compression set and a delayedrecovery from compression, in combination with a more difficultlycompressible layer, said last mentioned layer comprising a toughflexible polymeric matrix having a plurality of substantially unbrokenrigid hollow bodies embedded therein the arrangement being such thatunder the action of deforming forces deformation of the layers takesplace sequentially.
 2. An energy-absorbing material as claimed in claim1 having an outer skin resistant to abrasion and weathering.
 3. Thematerial claimed in claim 2 wherein the outer skin is polyurethane. 4.The material claimed in claim 2 wherein the outer skin has a thicknessof 1/16 to 1/2 inch.
 5. The material claimed in claim 1 mounted on arigid backing member.
 6. The combination of claim 5 wherein the moredifficulty compressible layer is adjacent to the backing member.
 7. Thematerial claimed in claim 1 wherein the more difficulty compressiblelayer contains 10 to 60% of said rigid hollow bodies by weight.
 8. Thematerial claimed in claim 1 wherein said bodies have an average diameterin the range 50 to 400 microns.
 9. The material claimed in claim 1wherein the elastomeric layer and the more difficultly compressiblelayers have respective thickness in the range 3 to 6 inches.
 10. Thematerial claimed in claim 1 wherein said matrix comprises apolyurethane.
 11. The material claimed in claim 1 wherein the elastomerlayer is composed of a composition which comprises a flexiblepolyurethane of essentially linear structure containing unsatisfiedhydroxyl groups, and having a compression set less than 15%, anelongation at break of at least 500%, and a recovery which is delayedafter compression by at least 0.7 sec.
 12. The material claimed in claim1 wherein the elastomer layer is composed of a composition having a lowcompression set and a delayed recovery from compression comprising thereaction product of a substantially linear polyol having hydroxyl endgroups and a molecular weight in the range 6000 to 12,000, and anisocyanate in less than stoichiometric amount, the elastomer containingunsatisfied OH groups.
 13. The material claimed in claim 1 wherein saidbodies are composed of a phenolic resin.
 14. The material claimed inclaim 1 wherein said bodies are composed of glass.
 15. The materialclaimed in claim 1 wherein said bodies are composed of silica.
 16. Thematerial claimed in claim 1 wherein said bodies are composed of carbon.